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35 Watt 1-FET Class E AM Transmitter

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Below are pictures of my 1-FET 35-watt Class E transmitter. It uses Steve Cloutier's (WA1QIX) new prototype Class H modulator board, and a single rail (2 MOSFETs) high voltage / high current Class H modulator output stage (mounted separately on a heat sink).
 
It is designed to be very easy to build and operate... and more details will be posted from time-to-time.
 
With a single RF FET (QFET) the problems associated with stabilizing the behavior of a multi-FET class E rig are largely avoided.
 
The main power supply is located on the underside of the chassis. The "wall plug" power consumption of the rig is about 90 watts. It requires ~5 watts of unmodulated RF drive... and I am using it in conjunction with an FT-1000 MkV transceiver that easily puts out 5 watts of continuous carrier for this purpose.
 
As an update: I have recently modified the input circuit to include a 100 ohm, 10 watt swamping resistor in parallel with the remainder of the input circuit. With the addition of this resistor, I find it easier to maintain a low input SWR. The drive power remains at ~5 watts. I use a 10:2 toroidal transformer to drive the gate of the single rf FET. The series input coil (on the 10-turn primary side of the transformer) resonates (approximately) with the input capacitance of the rf FET... and this results in a 12 volt peak sinousoidal signal on the gate of the rf FET.  Note that the 5 watt drive signal produces a sine wave with approximately 20 volts of peak voltage across the swamping resistor. With the 10:2 transformer, one might expect only 4 volts (20/5) on the gate of the FET. However, the series inductor, resonating with the FET input capacitance, results in the needed 12 volts of peak  voltage on the FET gate.

view1.jpg

In the photo below you can see the front side of Steve's (WA1QIX) new prototype class H modulator board. It includes: the low power audio amplifier and phase inverter stages, the stages that offset the d.c. reference level of the audio signal to "float" at the proper voltage level for driving the high voltage / high current stage (which is mounted on the heat sink),  the components for the +/- 12 volt (both non-floating and floating)  and +5 volt regulated supplies (except the transformers, which are mounted below the chassis), a circuit for detecting clipping on negative peaks of modulation, etc.

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35watttop.jpg

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From the front, you can see the output tuning coil on the left (20 turns of black, insulated #10 stranded wire on a 2.25 inch outside diameter PVC pipe), the input tuning coil on the right (27 turns of red, insulated #18 solid wire on a 1.25 inch outer diameter PVC pipe), the heat sink mounted on the back of the 12 x 8 x 3 inch chassis, and the tuning and loading capacitors in the middle. You can also see one edge and the back side of the new, prototype WA1QIX class H modulator board.

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closeup.jpg

In the picture above you can see a closeup view of some of the components that are mounted on the heat sink. Note that the FETs are attached to the heat sink with #4 screws that fit into holes that have been tapped into the heat sink. The 3 leads of each FET are connected to terminals on barrier strips. This makes it easy to change out an FET (which, hopefully, you won't have to do). The source lead of the RF FET (QFET) has an extra wire that is soldered on to it, close to the body of the device. The other end of this wire is connected to the secondary of the gate drive transformer. This minimizes the length of the portion of the source circuit path that is shared by the gate current and the drain current. Thus, the QFET has, in effect, 4 leads... that connect to 4 separate terminals on the corresponding barrier strip: gate, drain, source, and the auxilliary source lead.

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160meter.jpeg

This is my 1-FET rig after I modified it for 160 meters. I changed the output coil to about twice its previous number of turns, I changed the input coil to about 80 turns, I added a 100 ohm 10 watt input circuit swamping resistor, I changed the input transformer from 5:1 to 10:2, I changed the output transformer from 1:2 to 2:3, and I added 1500 pF of fixed capacitance across the output loading capacitor. The metal plate on the right hand side was added to physically protect the modulator printed circuit board.

FETClassE.jpg

This is my 1-FET rig after I , again, modified it for 160 meters. I removed the previous input circuit: i.e., the input coil L1, the input matching transformer, and the swamping resistor. I replaced it with an IXYS IXDD414CI driver; which I mounted, along with some associated, passive components, on top of the Q-FET. The IXDD414 requires only a logic level input to switch it on and off, and it supplies plenty of current to switch the gate of the QFET.  I use an ~5 volt RF input signal (sine wave)  superimposed on a small, positive DC offset (~ 1.25 volts) to drive it... but some people prefer use a comparator to interface between the RF input signal and the IXYS IXDD414 device.
 
I also modfied the output circuit to employ more capacitance and less inductance... by placing 300pF of fixed capacitance in parallel with the variable tuning capacitor.

powersplitter.jpg

Above is a drawing of a power splitter that I built to enable my 1-FET class E rig to drive a grounded grid linear amplifier... without dropping the voltage on the FET or mistuning the FET output circuit.
 
The 1:2 ferrite transformer converts the amplifier input impedance (as seen from the primary of the transformer) to 12.5 ohms.
 
The 50 ohm dummy load adds to the 12.5 ohm amplifier load, in series... producing a 62.5 ohm load on the 1-FET FET rig.
 
Meanwile, the input to the splitter, the output to the dummy load, and the output to the amp are all referenced to ground.
 
I built this in a small "Bud box" with three S0-239 connectors mounted on it... and the transformer inside. I used #18 insulated wire for the primary and the secondary windings on the 1:2 transformer. 
 
If the primary (input-to-dummy load) is 1 turn, and the secondary is 2 turns... then 80% of the power goes to the dummy load, and 20% of the power goes to the amplifier.
 
If the primary (input-to-dummy load) is 2 turns, and the secondary is 3 turns... then 69% of the power goes to the dummy load, and 31% of the power goes to the amplifier